U.S. patent application number 12/464733 was filed with the patent office on 2010-04-29 for radiation shielding members including nano-particles as a radiation shielding material and method for preparing the same.
This patent application is currently assigned to Korea Atomic Energy Research Institute. Invention is credited to Jinwoo JUNG, Jaewoo KIM, Byungchul LEE, Hee Min LEE, Min-Ku LEE, Sang Hoon LEE, Chang Kyu RHEE, Young Rang UHM.
Application Number | 20100102279 12/464733 |
Document ID | / |
Family ID | 42116593 |
Filed Date | 2010-04-29 |
United States Patent
Application |
20100102279 |
Kind Code |
A1 |
KIM; Jaewoo ; et
al. |
April 29, 2010 |
RADIATION SHIELDING MEMBERS INCLUDING NANO-PARTICLES AS A RADIATION
SHIELDING MATERIAL AND METHOD FOR PREPARING THE SAME
Abstract
Disclosed is a radiation shielding member having improved
radiation absorption performance, including 80.0.about.99.0 wt % of
a polymer matrix or metal matrix and 1.0.about.20.0 wt % of a
radiation shielding material in the form of nano-particles having a
size of 10.about.900 nm as a result of pulverization, wherein the
radiation shielding material is homogeneously dispersed in the
matrix through powder mixing or melt mixing after treatment with a
surfactant which is the same material as the matrix or which has
high affinity for the matrix. A preparation method thereof is also
provided. This radiation shielding member including the
nano-particles as the shielding material further increases the
collision probability of the shielding material with radiation,
compared to conventional shielding members including
micro-particles, thus reducing the mean free path of radiation in
the shielding member, thereby exhibiting superior radiation
shielding effects. At the same density, the shielding member has
reduced thickness and volume and is thus lightweight. The porosity
of the shielding member is minimized, thereby preventing the
deterioration of shielding effects and properties of the shielding
member and realizing applicability in spent fuel managing
transport/storage environments and the like.
Inventors: |
KIM; Jaewoo; (Daejeon,
KR) ; UHM; Young Rang; (Daejeon, KR) ; LEE;
Byungchul; (Daejeon, KR) ; JUNG; Jinwoo;
(Seoul, KR) ; RHEE; Chang Kyu; (Daejeon, KR)
; LEE; Min-Ku; (Daejeon, KR) ; LEE; Hee Min;
(Seoul, KR) ; LEE; Sang Hoon; (Daejeon,
KR) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Assignee: |
Korea Atomic Energy Research
Institute
Daejeon
KR
|
Family ID: |
42116593 |
Appl. No.: |
12/464733 |
Filed: |
May 12, 2009 |
Current U.S.
Class: |
252/478 |
Current CPC
Class: |
G21F 1/08 20130101; G21F
1/10 20130101 |
Class at
Publication: |
252/478 |
International
Class: |
G21F 3/00 20060101
G21F003/00; G21F 1/10 20060101 G21F001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 29, 2008 |
KR |
10-2008-0106438 |
Claims
1. A shielding member having improved radiation absorption
performance, comprising 80.0.about.99.0 wt % of a polymer matrix or
metal matrix and 1.0.about.20.0 wt % of a radiation shielding
material which is provided in a form of nano-particles having a
size of 10.about.900 nm as a result of pulverization, to increase a
collision probability with radiation, wherein the radiation
shielding material is homogeneously dispersed in the polymer matrix
or metal matrix through powder mixing or melt mixing after surface
treatment with a surfactant which is a same material as the polymer
matrix or metal matrix or which has high affinity for the polymer
matrix or metal matrix.
2. The shielding member as set forth in claim 1, wherein the
radiation is neutrons, gamma rays or X-rays.
3. The shielding member as set forth in claim 1, wherein when the
radiation is neutrons, the nano-particles comprise any one selected
from the group consisting of boron, lithium, gadolinium, samarium,
europium, cadmium and dysprosium, a compound thereof, or a mixture
thereof.
4. The shielding member as set forth in claim 1, wherein when the
radiation is gamma rays or x-rays, the nano-particles comprise any
one selected from the group consisting of lead, iron and tungsten,
a compound thereof, or a mixture thereof.
5. The shielding member as set forth in claim 1, wherein the
polymer matrix comprises any one or more selected from the group
consisting of polyvinylalcohol (PVA), polyethylene (PE), high
density polyethylene (HDPE), low density polyethylene (LDPE),
epoxy, and any one or more rubber selected from the group
consisting of synthetic rubber, natural rubber, silicone-based
rubber and fluorine-based rubber.
6. The shielding member as set forth in claim 1, wherein the metal
matrix comprises any one or more selected from the group consisting
of stainless steel, aluminum, titanium, zirconium, scandium,
yttrium, cobalt, chromium, nickel, tantalum, molybdenum and
tungsten, or an alloy thereof.
7. The shielding member as set forth in claim 1, wherein the
nano-particles are subjected to the surface treatment with the
surfactant which is the same material as the polymer matrix or
metal matrix or which has high affinity for the polymer matrix or
metal matrix, in conjunction with re-pulverization for preventing
the nano-particles from re-growing due to aggregation.
8. The shielding member as set forth in claim 1, wherein the
pulverization is performed through ball milling.
9. The shielding member as set forth in claim 7, wherein the
re-pulverization is performed through ball milling.
10. The shielding member as set forth in claim 1, wherein the
surfactant which is the same material as the polymer matrix or
which has high affinity for the polymer matrix comprises any one or
more selected from the group consisting of polyvinylalcohol (PVA),
polyethylene (PE), epoxy, and rubber, and the surfactant which is
the same material as the metal matrix or which has high affinity
for the metal matrix comprises any one or more selected from the
group consisting of stainless steel, aluminum, tungsten, titanium,
and nickel.
11. The shielding member as set forth in claim 1, wherein the
radiation shielding material in the form of the nano-particles
reduces a thickness and a volume of the shielding member.
12. The shielding member as set forth in claim 1, wherein the
shielding member is used for one or more selected from the group
consisting of anti-radiation clothes, spent fuel managing
transport/storage environments, spent fuel reprocessing facilities,
radiation facilities including accelerators, transport/storage
casks of radioactive material, cosmic radiation shields including
space shuttles and satellites, and military radiation shields.
13. A method for preparing the shielding member having improved
radiation absorption performance of claim 1, comprising:
pulverizing a radiation shielding material to nano-particles;
mixing the radiation shielding material in a form of the
nano-particles with a surfactant which is a same material as a
polymer matrix or metal matrix or which has high affinity for the
polymer matrix or metal matrix, thus realizing surface coating, and
simultaneously performing re-pulverization; and homogeneously
dispersing the radiation shielding material in the form of the
nano-particles in the polymer matrix or metal matrix.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This patent application claims the benefit of Korean Patent
Application Nos. 10-2008-0106438 filed Oct. 29, 2008, the contents
of which are incorporated herein by reference. A claim of priority
to all, to the extent appropriate is made.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a radiation shielding
members including nano-particles as a radiation shielding material
and to a method for preparing the same.
[0004] 2. Description of the Related Art
[0005] Radiation is largely classified into ionizing radiation and
non-ionizing radiation, while radiation typically designates
ionizing radiation in general.
[0006] Ionizing radiation includes alpha rays, beta rays, protons,
neutrons, gamma rays and X-rays, which cause ionization when
passing through the matter, and is specifically divided into direct
ionizing radiation and indirect ionizing radiation. Examples of
direct ionizing radiation include alpha rays, beta rays and
protons, which have an ability to directly ionize the matter, and
examples of indirect ionizing radiation include X-rays, gamma rays,
and neutrons, which have no ability to directly ionize the matter
but are capable of indirectly ionizing the matter through
interaction with the matter.
[0007] Non-ionizing radiation whose energy is relatively low to
such an extent that charged ions are not produced or an ionization
probability is very low when passing through the matter, and
examples thereof include infrared rays, visible rays, and UV
rays.
[0008] Alpha rays are absorbed and blocked by a material having a
thickness comparable to that of a sheet of paper, and may be
instantly stopped in the air, thus obviating a need to be
additionally shielded. The beta rays are known to have energy lower
than that of the alpha rays in most cases and may be halted even by
a thin aluminum foil or a plastic sheet.
[0009] Gamma rays whose energy is greater than that of the X-rays
are electromagnetic waves generated from nuclear disintegration or
transmutation, and have great penetrating power. Such gamma rays
and X-rays may be blocked with concrete or a high-density metallic
material such as iron or lead. In the case where the metallic
material is used, problems in which the weight of the shielding
member is undesirably increased owing to the high density of the
metallic material incur.
[0010] Neutrons are generated due to nuclear disintegration or
fission and are in an uncharged state. In the case of fast
neutrons, however, energy is high to the level of 1 MeV or higher,
and thus, in order to decelerate the fast neutrons, a material
containing a large amount of hydrogen having a mass similar to that
of a neutron may be used in combination. Further, there is required
a shielding member containing a neutron absorbing material for
absorbing thermal neutrons having low energy (.about.0.025 eV)
resulting from the deceleration of the fast neutrons.
[0011] In particular, gamma rays, X-rays or neutrons directly act
on atoms or molecules, thus changing the main structure of DNA or
proteins. When this type of radiation acts on the generative cell
of a living organism, a probability for inducing mutation to thus
bring about malformation and malfunction may be increased. In the
case where this type of radiation acts on the adult organism, a
disease such as cancer may be caused. Moreover, thermal neutrons
make the surrounding material radioactive to thus pollute the
surrounding environment with radioactivity. Hence, the area to
which radiation is applied essentially requires a radiation
shielding member able to shield gamma rays, X-rays or neutrons
harmful to the human body and the environment.
[0012] Conventionally, gamma rays or X-rays shielding member is
known to be imparted with shielding effects by using a material
containing iron, lead, and concrete. On the other hand, a neutron
shielding member is known to be a mixture of a polymer or metal
matrix and a compound including a material having a large thermal
neutron absorption cross-section, such as boron, lithium and
gadolinium having the ability to absorb thermal neutrons. For
example, Korean Patent Publication No. 10-2006-0094712 discloses a
shielding member using high-density polyethylene as a polymer
matrix in which boron known to absorb thermal neutrons and lead
known to decay gamma rays are mixed together in order to be easily
processed and shield from both neutrons and gamma rays. However,
the above patent does not recognize the fact that the particle size
of the radiation shielding material has a great influence on
radiation shielding performance.
[0013] To date, the performance of the radiation shielding member
is known to be determined merely by the properties of radiation
shielding material (depending on absorption cross-section in the
case of neutrons, or depending on the decay constant in the case of
gamma or X-rays), the amount of radiation shielding material in the
matrix, and the thickness of the shielding member. The particle
size of the radiation shielding material is not known to greatly
affect the radiation shielding performance. Further, there is no
report related to the preparation of a radiation shielding member
using homogeneous dispersion of a radiation shielding material in
the form of nano-particles in a polymer matrix.
SUMMARY OF THE INVENTION
[0014] Leading to the present invention, thorough research carried
out by the present inventors aiming to solve the problems
encountered in the related art, resulted in the finding that
nano-particles may be introduced as a radiation shielding material,
thus increasing the collision probability of the radiation
shielding material in the form of nano-particles with incident
radiation in the shielding member, thereby increasing radiation
shielding effects, and as well, the thickness and volume of the
shielding member may be decreased compared to shielding members
including particles having a size on at least the micro-scale as a
shielding material, such that the weight of the shielding member
may be reduced and the porosity of the shielding member may be
minimized, thereby preventing the shielding effects and the
properties of the shielding member from deteriorating due to the
presence of pores and enabling the radiation shielding member to be
usefully employed as a neutron absorber in spent fuel managing
transport/storage environments and the like.
[0015] An object of the present invention is to provide a radiation
shielding member including nano-particles as a radiation shielding
material, which can exhibit superior radiation shielding effects,
is lightweight, and can prevent the deterioration of the properties
of the shielding member.
[0016] Another object of the present invention is to provide a
method of preparing the radiation shielding member including
nano-particles as a radiation shielding material.
[0017] In order to accomplish the above objects, the present
invention provides a radiation shielding member and a method for
preparing the same, by homogeneously dispersing a radiation
shielding material in the form of nano-particles in a polymer
matrix or a metal matrix and then performing molding.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1(a) shows a scanning electron microscope (SEM) image
of the micro-B.sub.2O.sub.3/polyvinylalcohol (PVA) composite of
Comparative Example 1, and FIG. 1(b) shows an SEM image of the
nano-B.sub.2O.sub.3/PVA composite of Example 1;
[0019] FIG. 2(a) shows a transmission electron microscope (TEM)
image of the micro-B.sub.2O.sub.3/PVA composite of Comparative
Example 1, and FIG. 2(b) shows a TEM image of the
nano-B.sub.2O.sub.3/PVA composite of Example 1;
[0020] FIG. 3(a) shows the Monte Carlo N-particle (MCNP) pixel
array of 300 .mu.m boron oxide, and FIG. 3(b) shows the MCNP pixel
array of 0.5 .mu.m boron oxide, which are the concept of the
particle size-dependent MCNP simulation;
[0021] FIG. 4 is a graph showing the radiation shielding efficiency
using the MCNP simulation (particle size of the boron compound: 300
.mu.m (.quadrature.), 0.5 .mu.m (.smallcircle.), and 10.sup.-15 m
(.DELTA., nucleus size in a conventional MCNP));
[0022] FIG. 5 is a graph showing the shielding efficiency of the
radiation shielding material (boron content: 2.5 wt %) (Example
1(.smallcircle.), Comparative Example 1(.quadrature.)); and
[0023] FIG. 6 is a graph showing the shielding efficiency of the
radiation shielding material (boron content: 1.0 wt %) (Example
2(.smallcircle.), Comparative Example 2(.quadrature.)).
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present invention provides a radiation shielding member
prepared by homogeneously dispersing a radiation shielding material
in the form of nano-particles in a polymer matrix or metal
matrix.
[0025] Hereinafter, a detailed description will be given of the
present invention.
[0026] The radiation shielding member according to the present
invention includes a polymer matrix or a metal matrix and a
radiation shielding material in the form of nano-particles having a
size of 10.about.900 nm as a result of pulverization, the radiation
shielding material being homogeneously dispersed in the matrix. The
radiation shielding material in the form of nano-particles may
increase the collision probability with incident radiation in the
shielding member. Accordingly, the mean free path of the collided
radiation may be decreased, thus increasing a probability of
absorbing (and decaying) the radiation, consequently effectively
shielding the radiation.
[0027] The particle size of the radiation shielding material is
regarded as an important factor for increasing the collision
probability between the incident radiation and the shielding
material to thus increase the shielding efficiency. If the particle
size is less than 10 nm, it is difficult to prepare the
nano-particles. Conversely, if the particle size exceeds 900 nm,
the collision probability is reduced in proportion to the exceeding
thereof, thus making it difficult to attain the effective radiation
shielding efficiency of nano-particles. Such nano-particles may be
obtained by mechanically pulverizing a radiation shielding material
having a particle size ranging from tens to hundreds of .mu.m using
a mechanical activation process by means of a ball mill.
[0028] The amount of the radiation shielding material in the form
of nano-particles contained in the shielding member according to
the present invention may be set to 1.0.about.20.0 wt % depending
on the shielding purpose. If the amount is less than 1.0 wt %, the
radiation shielding effects are reduced. Conversely, if the amount
exceeds 20.0 wt %, the shielding efficiency may be increased but it
is difficult to homogeneously disperse the shielding material in
the polymer matrix or metal matrix and the weight of the shielding
member is remarkably increased.
[0029] Also, the amount of the polymer matrix or metal matrix
according to the present invention may be set to 80.0.about.99.0 wt
%. If the amount is less than 80.0 wt %, the deceleration
efficiency of fast neutrons is lowered. Conversely, if the amount
exceeds 99.0 wt %, the amount of radiation shielding material is
decreased, undesirably lowering the shielding efficiency.
[0030] Further, the radiation shielding member according to the
present invention may be molded to have a porosity of at most 5%.
The presence of pores in the shielding member deteriorates the
properties of the shielding member and as well impedes the
improvement in the radiation shielding effects. Therefore, it is
preferred that the radiation shielding member have a porosity as
low as possible.
[0031] Examples of the radiation to be shielded by the radiation
shielding member according to the present invention may include
neutrons, gamma rays or X-rays.
[0032] In the case where the radiation to be shielded is neutrons,
the nano-particles may include boron, lithium, gadolinium,
samarium, europium, cadmium, dysprosium, a compound thereof, or a
mixture thereof, having a large thermal neutron absorption
cross-section. The neutron-absorbing material may be selected
depending on the end use or the type of matrix. Particularly useful
is boron or a boron compound. Examples of the boron compound may
include B.sub.2O.sub.3, B.sub.4C, Na.sub.2B.sub.4O.sub.7, BN,
B(OH).sub.3 and the like.
[0033] In the case where the radiation to be shielded is gamma or
X-rays, the nano-particles may include lead, iron, tungsten, a
compound thereof, or a mixture thereof, having a high density.
[0034] Also, the shielding member according to the present
invention includes the polymer matrix or metal matrix in which the
radiation shielding material is dispersed. It is more desirable
that the polymer matrix or metal matrix be capable of facilitating
the molding to a final shielding member, minimizing the porosity
upon mixing with the nano-particles, and additionally exhibiting
radiation shielding effects.
[0035] Examples of the polymer matrix include, as a polymer
effective for decelerating fast neutrons thanks to a high hydrogen
density, polyvinylalcohol (PVA), polyethylene (PE), high-density
polyethylene (HDPE), low-density polyethylene (LDPE), epoxy, and
rubber including synthetic rubber, natural rubber, silicone-based
rubber and fluorine-based rubber, and ones mixed thereof. In
particular, polyethylene series are useful in terms of hydrogen
atom content.
[0036] Examples of the metal matrix include, being metals of high
density, stainless steel, aluminum, titanium, zirconium, scandium,
yttrium, cobalt, chromium, nickel, tantalum, molybdenum, tungsten,
and alloys thereof.
[0037] The radiation shielding material in the form of
nano-particles according to the present invention may be dispersed
in the polymer matrix or metal matrix through powder mixing or melt
mixing. As such, it is important to homogeneously disperse the
radiation shielding material in the form of nano-particles in the
polymer matrix or metal matrix. This is because the radiation
shielding effects of the shielding member should be uniformly
imparted to the entire shielding member.
[0038] In the case of using the powder mixing process, there is no
difficulty in homogeneously dispersing the nano-particles. However,
in the case of using the melt mixing process, the radiation
shielding material in the form of nano-particles may aggregate and
thus be difficult to homogeneously disperse. To solve the above
problems, the nano-particles may be mixed with a surfactant which
is the same material as the polymer matrix or metal matrix or which
has high affinity for the polymer matrix or metal matrix so that
the nano-particles are coated for surface activation, before being
dispersed in the polymer matrix or metal matrix. In this way, when
the surface of the nano-particles having low affinity for the
matrix is coated with the material having high affinity for the
matrix, the affinity between the nano-particles and the matrix may
be increased, such that the nano-particles in the matrix do not
aggregate but are homogeneously dispersed in the entire matrix. In
the case where the matrix is a polymer, the same material as the
matrix may be optimally used as the surfactant. When such a
material cannot be used, polyvinylalcohol, polyethylene, epoxy or
rubber may be used. Also, in the case where the matrix is a metal,
stainless steel, aluminum, tungsten, titanium or nickel may be
used.
[0039] Also, with the goal of making the nano-particles more fine
and preventing the nano-particles from re-growing due to
aggregation, in order to provide more effective dispersion,
re-pulverization may be performed through ball milling. The
nano-particles thus coated may be forcibly stirred at high speed to
homogeneously disperse them in a liquid polymer matrix or metal
matrix.
[0040] The shielding member according to the present invention is
provided as a radiation shielding member having a predetermined
shape by subjecting a powder phase or a liquid phase in which the
shielding material is homogeneously dispersed in the polymer matrix
or metal matrix to typical molding and/or processing. As such, the
process used for the molding and/or processing typically includes
compression molding, injection molding, extrusion, and casting. In
this case, the porosity of the shielding member should be
controlled to the minimum.
[0041] In addition, the present invention provides a method for
preparing the shielding member having improved radiation absorption
performance, including pulverizing a radiation shielding material
to nano-particles (step 1); mixing the radiation shielding material
in the form of the nano-particles obtained in step 1 with a
surfactant which is the same material as the polymer matrix or has
high affinity for the polymer matrix or a surfactant which is the
same material as the metal matrix or has high affinity for the
metal matrix, thus realizing surface coating, and simultaneously
performing re-pulverization(step 2); and homogeneously dispersing
the radiation shielding material in the form of the nano-particles
obtained in step 2 in the polymer matrix or metal matrix (step
3).
[0042] Below, the method of preparing the radiation shielding
member according to the present invention is described in detail in
steps.
[0043] Step 1
[0044] Step 1 according to the present invention is a process of
mechanically activating the radiation shielding material, thus
preparing the nano-particles. The radiation shielding material may
include the aforementioned gamma/X-rays shielding material or
neutron shielding material. The mechanical activation may be
performed using a ball mill, and ball milling may be conducted at
500.about.1100 rpm for 5.about.30 min.
[0045] Step 2
[0046] Step 2 according to the present invention is a process of
subjecting the radiation shielding material in the form of
nano-particles obtained in step 1 to coating with a material having
high affinity for the polymer matrix or metal matrix, in
conjunction with re-pulverization, in order to provide for
homogeneous dispersion in the polymer matrix or metal matrix.
[0047] Upon melt mixing, the homogeneous dispersion of the
nano-particles in the matrix is not easy because of the properties
of the nano-particles. To solve this problem, in the present
invention, the coating of the nano-particles is conducted in such a
manner that the nano-particles are coated with the surfactant which
is the same material as the polymer matrix or metal matrix used in
the present invention or which has high affinity for the above
matrix, thus increasing affinity of the nano-particles for the
matrix so as to homogeneously disperse the nano-particles in the
matrix. The useful coating material includes the aforementioned
surfactant which is the same material as the polymer matrix or
metal matrix or which has high affinity for the above matrix. The
surface activation or coating of the nano-particles may prevent the
particles from re-growing due to aggregation. This effect may be
more effectively achieved by performing the pulverization procedure
at the same time as the coating process.
[0048] In this step, the solvent such as cyclohexane, toluene or
normal-hexane may be added with a surfactant for better
re-pulverizing and coating to the surface of nano-particles using a
wet ball-mill process. Or, for the case of already prepared
nano-particles, they may be surface-coated by stir mix with a
surfactant in the solvent such as cyclohexane, toluene or
normal-hexane.
[0049] Step 3
[0050] Step 3 according to the present invention is a process of
homogeneously dispersing the radiation shielding material in the
form of nano-particles obtained in step 2 in the polymer matrix or
metal matrix. The dispersed shielding member may be adequately
molded to impart the thickness and volume adapted for the end
use.
[0051] By the preparation method according to the present
invention, the thickness and volume of the shielding member are
reduced, leading to a lightweight radiation shielding member. The
shielding effects of the shielding member may be achieved as a
result of pulverizing the radiation shielding material to
nano-particles so that the collision probability of the
nano-particles with incident radiation in the shielding member is
increased to thereby reduce the mean free path of the radiation.
Unlike this, in order to accomplish the same shielding effects as
in the shielding member including the nano-particles by using
particles having a size on at least the micro-scale as a radiation
shielding material, because the collision probability with incident
radiation should be increased to thus increase the mean free path
of the radiation, the shielding material in the form of the
particles having a size on at least the micro-scale should be
contained in a relatively large amount in the shielding member,
consequently undesirably increasing not only the weight of the
shielding member but also the volume thereof, namely, the thickness
thereof. From this point of view, the radiation shielding member
according to the present invention can achieve a light weight, as
well as show superior shielding effects.
[0052] The radiation shielding member according to the present
invention may be efficiently used in fields requiring radiation
shielding effects, for example, anti-radiation clothes, spent fuel
managing transport/storage environments, spent fuel reprocessing
facilities, radiation facilities including accelerators,
transport/storage casks of radioactive material, cosmic radiation
shields (space shuttles, satellites, etc.), and military radiation
shields.
[0053] A better understanding of the present invention may be
obtained through the following examples, which are set forth to
illustrate, but are not to be construed to limit the present
invention.
EXAMPLE 1
Preparation of Neutron Shielding Member 1
[0054] Step 1. Preparation of Neutron Absorbing Nano-Particles
[0055] Commercially available boron oxide (B.sub.2O.sub.3, High
Purity Chemicals, Japan) having a particle size of 200.about.300
.mu.m was subjected to ball milling at 1000 rpm for about 10 min,
thus preparing boron compound nano-particles having a particle size
of 100.about.1000 nm.
[0056] Step 2. Surface Activation of Boron Compound
Nano-Particles
[0057] The boron compound nano-particles obtained in step 1 were
subjected to milling at 700 rpm for 60 min with the same amount of
PVA, thus reducing the particle size and surface activating
(coating) the boron compound nano-particles with PVA. The surface
activation of the nano-particles can prevent the increase in the
particle size as they collide each other. Thereby, the particle
size could be advantageously maintained in the nano scale. In
accordance therewith, the average particle size of the boron
compound particles thus obtained was 210 nm.
[0058] Step 3. Dispersion of Surface-Activated Boron Compound
Nano-Particles in Polymer Matrix and Molding
[0059] The nano-powder in which the boron compound nano-particles
containing 2.5 wt % boron were surface-activated with an
appropriate amount of PVA was homogeneously dispersed in a PVA
polymer matrix and then heat-compressed to a thickness of 0.2 cm,
0.5 cm, 0.75 cm and 1 cm, thus preparing a radiation shielding
member including boron compound nano-particles.
EXAMPLE 2
Preparation of Neutron Shielding Member 2
[0060] A neutron shielding member was prepared in the same manner
as in Example 1, with the exception that the boron compound
nano-particles surface-activated with an appropriate amount of PVA,
used in step 3, had a boron content of 1.0 wt %.
EXAMPLE 3
Preparation of Neutron Shielding Member 3
[0061] Surface-activated B.sub.4C nano-powder (average particle
size: about 50 nm) was prepared in the same manner as in steps 1
and 2 of Example 1, with the exception that B.sub.4C was used as
the radiation shielding material. Thereafter, the nano-powder thus
prepared was melt mixed with a HDPE polymer matrix with forcible
stirring, and then injection molded, thus preparing a radiation
shielding member. Thus, when using the present process, the
nano-particles were confirmed to be homogeneously dispersed not
only in the powder mixing but also in melt mixing.
COMPARATIVE EXAMPLE 1
Preparation of Neutron Shielding Member Using Boron Compound
Micro-Particles 1
[0062] A neutron shielding member containing a neutron shielding
material in the form of micro-particles was prepared in the same
manner as in Example 1, with the exception that, in step 3,
commercially available boron oxide (B.sub.2O.sub.3, High Purity
Chemicals, Japan) having a size of 200.about.300 .mu.m was used
instead of the boron compound nano-particles.
COMPARATIVE EXAMPLE 2
Preparation of Neutron Shielding Member Using Boron Compound
Micro-Particles 2
[0063] A neutron shielding member containing a neutron radiation
shielding material in the form micro-particles was prepared in the
same manner as in Example 2, with the exception that, in step 3,
commercially available boron oxide (B.sub.20.sub.3, High Purity
Chemicals, Japan) having a size of 200.about.300 .mu.m was used
instead of the boron compound nano-particles.
COMPARATIVE EXAMPLE 3
Commercially Available Neutron Shielding Member
[0064] A commercially available neutron shielding member (Nelco,
USA) in which boron compound (B.sub.2O.sub.3) particles having a
size of 200.about.300 .mu.m with 9.0 wt % boron were dispersed in a
polyurethane matrix was used.
COMPARATIVE EXAMPLE 4
Commercially Available Neutron Shielding Member
[0065] A commercially available neutron shielding member (Nelco,
USA) in which boron compound (B.sub.2O.sub.3) particles having a
size of 200.about.300 .mu.m with 5.0 wt % boron were dispersed in a
HDPE matrix was used.
EXPERIMENTAL EXAMPLE 1
Observation of Boron Nano-Particles Dispersed in Radiation
Shielding Member
[0066] In order to evaluate the dispersion state of the boron
compound nano-particles, the neutron shielding member of each of
Example 1 and Comparative Example 1 was observed using SEM and TEM.
The results are shown in FIGS. 1(a), 2(a) for Comparative Example 1
and FIGS. 1(b) and 2(b) for Example 1.
[0067] As shown in FIGS. 1(a), 1(b), 2(a) and 2(b), compared to the
shielding member of Comparative Example 1 including
micro-particles, the boron compound nano-particles could be seen to
be homogeneously dispersed in the PVA matrix.
EXPERIMENTAL EXAMPLE 2
Simulation of Radiation Shielding Efficiency Depending on Particle
Size of Radiation Shielding Material Using MCNP Transport Code
[0068] The neutron absorption probability by the shielding member
in which 300 .mu.m boron oxide compound particles (a) including
boron nuclei having a size of about 10.sup.-15 m were homogeneously
dispersed in HDPE and by the shielding member in which 0.5 .mu.m
boron oxide compound particles (b) having the same boron nuclei
were homogeneously dispersed in HDPE was simulated using MCNP.
[0069] Conventional MCNP simulation is unable to calculate the
radiation shielding efficiency depending on the particle size. So,
in the present invention, simulation was carried out by
respectively locating the boron compound particles in the centers
of pixels such that the boron oxide compound particles had a size
of 300 .mu.m and the boron content was 2.5 wt % and then
standardizing these pixels to an array. Also in the case of 0.5
.mu.m boron oxide compound particles, the simulation was performed
in the same manner. The basic simulation concept is shown in FIGS.
3(a) and 3(b).
[0070] The above results were compared with simulation results
using the conventional MCNP method (because the simulation was
conducted under an assumption in which the boron nuclei were
homogeneously dispersed, the particle size was set to about
10.sup.-15 m) depending only on the microscopic neutron absorption
cross-section of the shielding material and the boron content
thereof. The results of neutron absorption efficiency using the
particle size-dependent MCNP simulation and the conventional MCNP
simulation are shown in FIG. 4.
[0071] As shown in FIG. 4, the radiation absorption efficiency of
the shielding member (.smallcircle.) including 0.5 .mu.m boron
oxide compound particles was increased by about 25.about.75%, which
varies depending on the thickness of the shielding member, compared
to the shielding member (.quadrature.) including 300 .mu.m boron
oxide compound particles. The simulation results (.DELTA.) using
the conventional MCNP method exhibited a radiation shielding
efficiency increased by more than 50%, compared to the above
particle size-dependent simulation results. This is considered to
be because the conventional MCNP method supposes that the particle
size of the radiation shielding material is set to the respective
boron nuclei having a size of 10.sup.-15 m which are uniformly
distributed.
[0072] The MCNP simulation method depending on the particle size
according to the present invention may cause an experimental
measurement differences in comparison to the conventional MCNP
simulation. This is because whereas the conventional MCNP method
does not consider the particle size, the actual radiation shielding
member includes large shielding particles (boron compounds) in
which hundreds to tens of thousands of boron nuclei
agglomerate.
EXPERIMENTAL EXAMPLE 3
Measurement of Radiation Shielding Efficiency
[0073] The neutron shielding efficiency of Examples 1 and 2 and
Comparative Examples 1 to 4 was measured and calculated in
compliance with the following procedures.
[0074] The thermal neutron shielding efficiency may be calculated
using Equation 1 below.
I(t)=I.sub.oe.sup..SIGMA..sup.th.sup.t Equation 1
[0075] wherein I.sub.o is the incident neutron beam flux
(n/cm.sup.2/s) t is the thickness (cm) of the shielding member,
.SIGMA..sub.th is the macroscopic thermal neutron absorption
cross-section (cm.sup.-1) which is given as N.sigma. in which N is
a number density (number of atoms/cm.sup.3) of the neutron
shielding material and .sigma. is the microscopic thermal neutron
absorption cross-section (cm.sup.2) which is an intrinsic value of
the material and is experimentally measured. The mean free path
(.lamda..sub.th) of the neutron is represented by 1/.SIGMA..sub.th
as an inverse number of .SIGMA..sub.th.
[0076] By the use of the FCD (Four Circle Diffractometer) at a
Hanaro Center in the Korea Atomic Energy Research Institute, a
thermal neutron source having a wavelength of about 0.997 .ANG. and
a flux of about 6.6.times.10.sup.5 n/cm.sup.2/s was radiated onto
the shielding member for 10 sec. Then, using a He-3 proportional
counter as a detector spaced apart from the sample by about 2 m,
the number of neutrons passing through the shielding member
depending on the shield thickness and the content was subjected to
at least ten measurements, after which the measured values were
averaged.
[0077] As shown in FIG. 5, the neutron shielding member including
boron compound particles of 2.5 wt % boron had a tendency to
increase the shielding efficiency in proportion to the thickness
thereof. At the same thickness, the shielding efficiency of Example
1 (.smallcircle.) having smaller boron compound particles was
superior to that of Comparative Example 1 (.quadrature.).
[0078] As shown in FIG. 6, the shielding member including boron
compound particles of 1.0 wt % boron had a tendency to increase the
shielding efficiency in proportion to the thickness thereof, as in
the case shown in FIG. 5. At the same thickness, the shielding
efficiency of Example 2 (.smallcircle.) having smaller boron
compound particles was superior to that of Comparative Example 2
(.quadrature.).
[0079] From the ratio of the number of neutrons passed through the
shielding member to the number of incident neutrons, the thermal
neutron absorption cross-section (.SIGMA..sub.th) and the mean free
path (.lamda..sub.th) were calculated. The results are shown in
Table 1 below.
[0080] Consequently, in the case where the particle size was small,
the mean free path (.lamda..sub.th) was reduced by at least 15%,
thus increasing the neutron shielding efficiency.
TABLE-US-00001 TABLE 1 Thermal Neutron Absorption Cross-Section
& Mean Free Path Macroscopic Boron Thermal Neutron Thermal
Neutron Mean (wt %) Cross-Section, .SIGMA..sub.th (cm.sup.-1) Free
Path, .lamda. (cm) Ex. 1 2.5 1.72 0.58 Ex. 2 1.0 1.42 0.70 C. Ex. 1
2.5 1.49 0.67 C. Ex. 2 1.0 1.25 0.80 C. Ex. 3 9.0 2.21 0.45 C. Ex.
4 5.0 1.45 0.69
[0081] As is apparent from Table 1, Example 1 having the same boron
content as Comparative Example 1 had the macroscopic thermal
neutron absorption cross-section increased by about 15%, and
Example 2 having the same boron content as Comparative Example 2
had the macroscopic thermal neutron absorption cross-section
increased by about 14%. Also, as is apparent from Table 1, the
shielding member including 1.0 wt % nano-boron could show neutron
shielding performance similar to that of the shielding member
including 2.5 wt % micro-boron, thereby enabling the weight of the
shielding member to be reduced.
[0082] Further, as seen in Table 1, as commercially available
products from Nelco, USA, Comparative Examples 3 and 4 had the
boron content 3.6 times and 2 times respectively that of Example 1,
and 9 times and 5 times respectively that of Example 2.
Nevertheless, these comparative examples merely had the thermal
neutron absorption cross-section 1.28 times and 0.84 times
respectively that of Example 1 and 1.55 times and 1.02 times
respectively that of Example 2. From these results, compared to
Comparative Examples 3 and 4 including micro-particles, the neutron
shielding member of Examples 1 and 2 according to the present
invention had a much smaller amount of the radiation shielding
material, but could be seen to exhibit similar shielding effects
and in some cases superior effects.
[0083] In Comparative Example 3 in which many pores were present in
the shielding member due to the use of polyurethane as the polymer
matrix, the degree of improvement in the shielding effect was
insignificant despite the presence of a much greater amount of
boron compound compared to Examples 1 and 2. This is considered to
be because the shielding member including the polyurethane matrix
has 90% porosity and is thus reduced in shielding effects.
[0084] Therefore, even when the radiation shielding member of the
present invention includes a smaller amount of the radiation
shielding material compared to the conventional radiation shielding
member, superior radiation shielding effects versus the amount used
can be exhibited. Further, the lightweight radiation shielding
member can be realized.
[0085] As described hereinbefore, the present invention provides a
radiation shielding member including nano-particles as a radiation
shielding material and a preparation method thereof. According to
the present invention, the radiation shielding member in which the
radiation shielding material in the form of nano-particles is
homogeneously dispersed in a matrix can increase the collision
probability of the shielding material with radiation, compared to
conventional shielding members including, as a radiation shielding
material, particles of at least a micro-scale size. Hence, the mean
free path of the radiation in the shielding member is reduced, thus
exhibiting radiation shielding effects superior to conventional
radiation shielding members. As well, under a condition of the same
density, the shielding member according to the present invention
can have decreased thickness and volume, thus enabling the weight
of the shielding member to be reduced. Further, the porosity of the
shielding member can be minimized, thereby preventing the shielding
effects and the properties of the shielding member from
deteriorating attributable to the presence of pores and enabling
the shielding member according to the present invention to be
usefully employed in spent fuel managing transport/storage
environments and the like.
[0086] Although the preferred embodiments of the present invention
have been disclosed for illustrative purposes, those skilled in the
art will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
* * * * *